The guy is an experienced fighter, on top of it we all know that its extremely hard to achieve the admiral rank, ppl sure tend to underrate admirals time to time but the manga is there to prove them wrong, ofc he is holding back but still it doesnt downplay Sabo , he is an top tier but being weaker then an admiral who belongs in the general level of the original C3 admirals automatically makes him stronger then Sabo even then fujis motive is not to do anything to Sabo he definitely is hiding something about his true plan, and that is doflamingo being defeated while he use Sabo as an excuse cuz if Fuji would really want to help dofla he would have done that but he hates him in one way but doesnt have the authority to arrest him.
I always though Sabo was strong but the hype he got from fans was unreal I mean fanboys suck, Sabo is strong but he is far cry from admirals/yonko fighters.